Injection molding machines with respect to which the present invention pertains generally include an elongated feed screw which rotates and reciprocates in the cylindrical bore of an elongated barrel for creating and moving molten plastic material from an inlet end to an outlet end where it is injected from a nozzle to a mold cavity. The flow of plastic material from the feed screw is normally controlled by a check valve assembly. Ring-type check valve assemblies to prevent backflow of the liquid resin during injection are well known, as for example, in U.S. Pat. Nos. 3,438,393, 3,501,810, 3,590,439, 4,105,147, 4,377,180, 4,477,242 and 4,643,665.
In accordance with the prior art, the valve is closed immediately prior to injection; however, the art requires that start of the injection action causes an initial backflow of resin to push the sliding ring of the check valve to a closed position and hence to stop the backflow. A disadvantage of this procedure is that each time a variable amount of backflow actually takes place before the valve closes and injection proper can start. Also, it is an inherent disadvantage of this method that a precise measurement of the shot to be injected cannot be made.
For example, the '439 and '147 patents listed above disclose means to prevent the rotation of the ring in order to reduce its wear at the sealing surfaces; however, they continue to use backflow to close the valve.
It is, therefore, an object of the present invention to provide a check valve assembly for an injection molding machine for controlling the flow of plastic material from a feed screw which prevents backflow of the resin.
It is a further objective of the present invention to provide an assembly as aforesaid which does not rely on the start of the injection action to close the check valve assembly.
Still further objectives of the present invention is to provide an assembly as aforesaid which enables a precise measurement of the injection shot and which is simple and convenient to use.
Further objects and advantages of the present invention will appear hereinbelow.